Why concrete releases carbon and how to make it cleaner

HostYou look around any city and it's basically a world of gray. It's under our feet as sidewalks, it's the bones of our tall buildings, and it's even what keeps our bridges standing. It's so common that we kind of stop seeing it, but I have been reading that this one material is responsible for a massive chunk of the carbon going into the air. Why is something as simple as a mix of rock and water such a huge problem for the planet?

GuestIt's because of the sheer scale. Next to water, concrete is the most used material on earth. We use more of it than wood, steel, plastic, and aluminum put together. If the cement industry were its own country, it would be the third biggest polluter in the entire world, right behind China and the United States. And the really tricky part is that you can't just fix it by switching to a different fuel or using a big battery. The problem is actually baked into the very recipe we use to make it.

HostThat sounds like a tough spot to be in. If it's not just about the fuel we burn, what's actually happening inside the mix that makes it so dirty?

GuestWell, first we have to look at cement, which is the glue that holds concrete together. To make that glue, you need limestone. You take these massive chunks of rock and you shove them into a giant kiln, which is basically a spinning oven that gets hotter than lava. But here is the catch. Limestone is mostly made of calcium, but it's also holding onto a lot of carbon. When you heat that stone to those extreme levels, the stone actually splits apart. The calcium stays behind to become the glue, but the carbon is set free. It turns into gas and flies right out of the chimney.

HostSo even if we had a perfectly clean, electric oven that ran on sunshine, we would still be releasing all that carbon just by breaking the rocks?

GuestYou hit the nail on the head. About half of the carbon from a cement plant comes from the rocks themselves, not the fire. It's a chemical change. You're literally unmaking a stone and letting its ghost out into the sky. And then you have the other half of the problem, which is the heat. To get an oven that hot, you usually have to burn a lot of coal or gas. So you're getting hit from both sides. You're burning fuel to get the heat, and then the heat triggers a reaction in the rocks that releases even more gas.

HostThat feels like a double bind. If the chemistry itself is the problem, is there even a way to make it cleaner? I mean, we can't just stop building things.

GuestPeople are trying a few different paths. One of the most common ways right now is to just use less of that heavy glue. We can swap out some of the cement for other things that behave like it. For a long time, we used the ash left over from burning coal or the waste from making steel. It's basically recycling industrial trash into our roads. But as we move away from coal, that ash is getting harder to find. So now, researchers are looking at common clay. If you bake certain types of clay at a lower heat, you can use it to replace a big chunk of the cement. It makes the whole process much leaner.

HostIs that enough, though? It sounds like we're just making a bad process slightly less bad. Is there a version of this where we actually stop the gas from leaving the plant in the first place?

GuestThat's the big goal. There are companies now that are trying to catch the gas as it comes out of the chimney and then shove it right back into the wet concrete while it's being mixed. It's a bit like how you put bubbles into a soda. The carbon gas reacts with the wet mix and turns back into a solid mineral. It actually makes the concrete stronger, and that carbon is trapped there forever. It's not going back into the air unless you grind the whole building into dust and bake it all over again.

HostWait, if it makes the concrete stronger, why are we not doing that everywhere already? Is it just too expensive?

GuestThat's part of it. There's always a green price tag when you start out. But the bigger hurdle is actually the rules we have for building. If you're building a bridge that needs to last a hundred years, you're going to be very careful about changing the recipe. Most building codes are written around the old way of doing things. It takes a long time to prove to a city or a government that this new, low carbon mix is just as safe as the stuff we have been using since the Roman times. There's a lot of fear of being the first one to try a new recipe on a major project.

HostIt's funny you mention the Romans, because their concrete has lasted for thousands of years. Did they know something we do not?

GuestThey actually used volcanic ash, which reacted in a way that made the concrete get stronger over time, even when it was hit by seawater. We're actually looking back at those old recipes now to see if we can learn how to make stuff that lasts longer. If a building lasts two hundred years instead of fifty, you don't have to build it four times. That might be the simplest way to cut down on all that carbon.

HostIt seems like we're trying to turn a material that has been a major problem into something that might actually help us clean up the mess.

GuestThe dream is a world where our buildings act like a forest, where the very walls of your house are holding onto carbon instead of having released it into the sky.

HostThose gray sidewalks might eventually be the very thing that helps us keep the air clear.